1. Field of the Invention
This invention pertains to free wheeling tires particularly useful in a gravity roller conveyor, and the like having the ability to retard accelerated movement of the load carried by the wheels.
2. Discussion of the Prior Art
In many instances it is desireable to prevent increased rotational velocities of free-wheeling tires, wheels, or rollers. For example, in applications such as in mining operations, it is necessary that means be provided to guard against ore-bearing carts from running away and endangering property and personnel. Similarly, in applications where pallets and cargo are moved down an inclined path of free-wheeling rollers such as in gravity conveyors, the need to prevent high speeds of the pallet is always present.
Various devices to accomplish the above are found in the prior art. For example, U.S. Pat. No. 3,621,960 describes a gravity conveyor having rollers to retard the speed of the load. Each is provided with a tire formed of a polymer having a certain durometer hardness range. Durometer itself is only a measurement of the hardness of an elastomeric material and provides little insight into the appropriateness of that material under various temperatures or even at later times after extended operations. For example, since the elastic moduli of elastomers generally decreases as temperature increases, the durometer of a material at an initial time and temperature is of little value in predicting how an elastomer might perform under load at a later time where the temperature is elevated. It is also known that durometer readings may not accurately reflect the behaviour of elastomers throughout a tensile test.
The patentee points out that the surface material has high hysteresis properties, i.e., the ability to absorb energy upon deformation and convert it to heat. In other words, the energy lost in generating heat is derived from the energy used to rotate the tire. In applications where gravity is acting upon the loaded tire, the tire would otherwise steadily increase in rotational speed. The hysterisis characteristic of the tire offsets the affect of gravity such that the tire rotates at a constant speed. Hysterisis measurements of an elastomer, however, involve a complex set of variables. However, for purposes here, it can be said that hysterisis (H) and resilience (R) have the following relationship: EQU R=1-H
thus, the larger hysterisis of a material becomes, the less resilience it has. The heat generated must be dissapated quickly or ultimately the heated material behaves in a manner different than its initial behavior. The prior art, as for example the aforementioned U.S. Pat. No. 3,621,960, does not appear to have recognized the problem of excess heat generation which deleteriously affects the operation of constant speed wheels. It follows that no solution has been sought, much less found.
As mentioned before, resiliency and hysterisis of elastomeric materials are inversely related. Although neither can be determined exactly, emperical relationships can be employed which adequately predict the values at a particular temperature. A tire, wheel, or roller designed to maintain essentially constant rotational speeds under various loads must necessarily have a higher hysterisis value. It is desireable that the resiliency value throughout operation be essentially the same as the initial resilience value. This also fixes the hysterisis value.
Thus, it becomes a paramount object of the present invention to provide a rotating element structure which maintains essentially constant rotational speeds under load and which readily dissapates heat generated by deformation during operation.
Because it is necessary for apparatus such as gravity conveyors to control the speed of the descending load, still another object of the present invention is to provide a device which will limit the speed of an object moving thereover to a predetermined terminal velocity.
Applications employing speed control wheels have additionally been limited to using wheels of various compositions, each composition designed for a specific hysterisis effect and therefore a different speed control effect. From an economic standpoint, it would be highly desireable to have wheels which provide the desired speed control effects but where the composition of material remains unchanged. Thus, it is still further important object of the present invention to provide for rotating element structures where the hysterisis behaviour characteristics can be varied without changing the composition of the structure.